1. Field of the Invention
The present invention relates to surface shape measuring apparatuses and exposure apparatuses.
2. Description of the Related Art
The related art regarding a surface shape measuring apparatus and an exposure apparatus employing the former is described in connection with a semiconductor exposure apparatus which needs high accuracy in surface shape measurement.
When a microstructural semiconductor device or liquid crystal display device, such as a semiconductor memory or a logical circuit, is manufactured by using photolithography (printing) techniques, a projection exposure apparatus is used to project and transfer a circuit pattern, which is drawn on a reticle (mask), onto a wafer, etc. through a projection optical system.
In the projection exposure apparatus, a higher packing density of a semiconductor device causes a demand for projecting the circuit pattern on the reticle to the wafer for exposure with higher resolving power. A minimum critical dimension (resolution) transferrable in the projection exposure apparatus is proportional to the wavelength of light used in the exposure and is inversely proportional to the numerical aperture (NA) of the projection optical apparatus. Accordingly, as the exposure wavelength is set to a shorter value, higher resolving power is obtained. For that reason, a light source used in the projection exposure apparatus has recently been changed from an ultrahigh pressure mercury lamp (i.e., the g line (wavelength of about 436 nm) or the i line (wavelength of about 365 nm)) to a KrF excimer laser (wavelength of about 248 nm) or an ArF excimer laser (wavelength of about 193 nm), which emits light with a shorter wavelength. Practical use of immersion exposure is also studied. In addition, an even wider exposure region is demanded.
To meet those demands, a dominating exposure apparatus is changed from a step-and-repeat type (also called “stepper”) in which a substantially square exposure region is printed on a wafer at a reduction scale by one-shot exposure, to a step-and-scan type (also called “scanner”) in which an exposure region has a rectangular slit shape and a larger target area can be exposed with higher accuracy by relatively scanning a reticle and a wafer at a high speed.
In the scanner, during the exposure, before a predetermined position on the wafer reaches an exposure slit region, a wafer surface position (i.e., a position in the direction of an optical axis of a projection optical system, also called a focus) at the predetermined wafer position is measured by a surface position detecting unit in the form of a light oblique-incidence system. In accordance with a measurement result, correction is performed such that the wafer surface is aligned with the best focus position for the exposure when the predetermined wafer position is exposed.
In particular, a plurality of measurement points are set in the exposure slit region in the lengthwise direction (direction perpendicular to the scanning direction) of the exposure slit to measure not only the height (focus) of the wafer surface position, but also the tilt of the wafer surface. Many methods have been proposed to measure the focus and the tilt. As methods of measuring the wafer surface position, for example, Japanese Patent Laid-Open No. 06-260391 and U.S. Pat. No. 6,249,351 propose the use of an optical sensor. PCT Application Domestic Laid-Open No. 2006-514744 proposes the use of a gas gauge sensor configured to spray air to a wafer and to measure a wafer surface position. Another method of using an electrostatic capacitance sensor is also proposed.
Recently, however, with a tendency toward a shorter wavelength of the exposure light and a larger NA of the projection optical system, the focal depth has become very small, and accuracy demanded in aligning the wafer surface to be exposed with the best focus plane, i.e., the so-called focus accuracy, has been increased to an even higher level. In particular, measurement errors of the surface position detecting unit have become non-negligible even when the measurement errors are caused by an influence of a pattern on the wafer and a variation in thickness of a resist coated on the wafer.
Due to a variation in thickness of the resist, for example, a step-like level difference is generated near peripheral circuit patterns and scribe lines, though small in comparison with the focal depth, to such an extent as being significant to the focus measurement. Therefore, a tilt angle of the resist surface is increased, and among reflected lights detected by the surface position detecting unit, the reflected light from a rear surface of the resist is shifted from a specular reflection angle after being reflected or refracted. Further, due to a difference in density of patterns on the wafer, reflectance differs between a region where the patterns are dense and a region where the patterns are coarse. Thus, among the reflected lights detected by the surface position detecting unit, the reflected light from the rear surface of the resist is changed in reflection angle and reflection intensity, and a waveform obtained by detecting such reflected light becomes asymmetric and measurement errors are generated.
FIG. 19 illustrates a case where measurement light MM is illuminated to a substrate SB, which has reflectance differing in different regions, in an optical sensor proposed in Japanese Patent Laid-Open No. 06-260391. In the illustrated case, the measurement light MM is inclined at an angle A with respect to a boundary line between regions differing in reflectance such that the measurement is performed in a direction denoted by α′. FIG. 20 illustrates intensity distributions of the reflected lights at three cross-sections spaced from each other in a direction denoted by β′, i.e., at cross-sections AA′, BB′ and CC′. The reflected light has good symmetry at the cross-sections AA′ and CC′. At the cross-section BB′ including the regions differing in reflectance, the reflected light has an asymmetrical profile. In other words, the barycenter of the reflected light is shifted from a predetermined position and measurement errors are generated. Accordingly, the waveform of a signal detected by receiving the reflected light becomes asymmetrical and the contrast of the detected signal waveform deteriorates significantly, thus causing a difficulty in accurately measuring the wafer surface position. Such a difficulty results in a large defocus and a chip failure.
As described above, the intensity of the reflected light is changed due to interference generated by lights reflected from the front and rear surfaces of the resist depending on the patterns on the wafer. In some cases, therefore, it is difficult to accurately detect a position on the wafer surface by receiving the reflected light.
FIG. 23 illustrates the construction of a surface shape measuring apparatus disclosed in U.S. Pat. No. 6,249,351. The disclosed surface shape measuring apparatus includes a light source 101, a lens 103, a beam splitter 105, a reference mirror 130, a beam combiner 170 in the form of a diffraction grating, a lens 171, a lens 173, and a photoelectric conversion element 175. In that surface shape measuring apparatus, light is obliquely illuminated to a sample 360 and the shape of the sample 360 is determined from an interference signal detected by the photoelectric conversion element 175. The light received by the photoelectric conversion element 175 includes the reflected light from the front surface of a resist and the reflected light from the rear surface of the resist. This raises a difficulty in accurately measuring the shape of the resist front surface. FIG. 21 illustrates an interference signal obtained in the known apparatus, illustrated in FIG. 23, when the sample 360 is scanned by an actuator 397 in a direction perpendicular to the sample surface. The interference signal in FIG. 21 is obtained when measuring a sample which has no patterns on a wafer and which is coated with only a resist, as illustrated in FIG. 22. Because the received light includes not only the reflected light from the front surface of the resist but also the reflected light from the rear surface of the resist, the resulting interference signal is measured in such a state that the interference generated by the reflected light from the rear surface of the resist affects the interference generated by the reflected light from the front surface of the resist in a superimposed manner. This leads to a difficulty in accurately detecting height information of the resist front surface by using only the reflected light from the front surface of the resist. To measure the interference signal while separating the reflected lights from the front and rear surfaces of the resist, U.S. Pat. No. 6,249,351 proposes a method of increasing the reflectance at the front surface of the resist by increasing an incident angle upon the substrate. U.S. Pat. No. 6,249,351 says that the proposed method is effective in relatively intensifying the reflected light from the front surface of the resist on the substrate as compared with the reflected light from the rear surface of the resist.
However, when the substrate is made of Al or Cu and has high reflectance, the rear surface of the resist (i.e., the resist/substrate interface) has high reflectance to such an extent that the influence of the reflected light from the rear surface of the resist cannot be sufficiently suppressed even when the incident angle of the light upon the substrate is set to a large value. Accordingly, errors are generated in a value resulting from measuring the resist front surface.
Further, when a gas gauge sensor is used as described in PCT Application Domestic Laid-Open No. 2006-514744, specific problems arise in that fine particles mixed in gas are also sprayed onto the wafer, and that the gas gauge sensor cannot be used in an exposure apparatus operating in vacuum, e.g., an EUV (Extreme Ultraviolet) exposure apparatus using an extreme ultraviolet light, because a degree of vacuum deteriorates with the sprayed gas.